1. Field of the Invention
The present invention relates to an inductive load drive circuit. More specifically, the present invention relates to an inductive load drive circuit which is applied to a disc drive for a disc such as CD (Compact Disc), CD-ROM, MD (Mini Disc) and etc., and controls a current flowing an inductive load included in a pickup feeding motor, a spindle motor, a loading motor, a lens actuator and etc.
2. Description of the Prior Art
One example of conventional such a kind of inductive load drive circuit is shown in FIG. 4 and FIG. 5. In a case where the inductive load drive circuit shown in FIG. 4 is provided in the lens actuator of an optical pickup, excitation currents I.sub.A and I.sub.B flowing an inductive load L are switched in response to an input voltage V.sub.i based on a tracking control signal or a focus control signal. That is, the input voltage V.sub.i is amplified by a negative feedback amplifier 1 including an operational amplifier 2, whereby a drive voltage V.sub.O of the inductive load L is obtained. The excitation currents I.sub.A and I.sub.B having phases each delayed 90 degrees from the drive voltage V.sub.O flow through the inductive load L.
An output stage of the operational amplifier 2 is constituted as shown in FIG. 5. The drive voltage V.sub.O is outputted from an output terminal S1 in response to a control voltage V.sub.c applied to a base of a transistor T4. As shown in FIGS. 6A and 6B, the phases of the excitation currents I.sub.A and I.sub.B flowing through the inductive load L are respectively delayed 90 degrees from the drive voltage V.sub.O. In FIG. 6A, a positive excitation current I.sub.A flows into the inductive load L, and a negative excitation current I.sub.B flows from the inductive load L.
Furthermore, a push-pull circuit is constituted by transistors T3 and T1. Accordingly, when the excitation current I.sub.A flows into the inductive load L, the transistor T3 is turned-on and the transistor T1 is turned-off. On the other hand, when the excitation current I.sub.B flows from the inductive load L, the transistor T3 is turned-off and the transistor T1 is turned-on.
When the excitation current I.sub.A flows, a collector voltage of a transistor T4 becomes slightly higher than the drive voltage V.sub.O, whereby the transistor T3 is turned-on. In contrast, when the excitation current I.sub.B flows, the collector voltage of the transistor T4 becomes equal to the drive voltage V.sub.O, whereby the transistor T1 is turned-on by an idling loop composed of a constant-current source C1, diodes D1 and D2, and transistors T5 and T6. That is, the diodes D1 and D2 are inserted between the output terminal S1 and a base of the transistor T5, and forward drop voltages of the diodes D1 and D2 and base-emitter voltages between bases and emitters of the transistors T5 and T6 are equal to each other, respectively. Though the transistors T5 and T6 are not turned-on if the collector voltage of the transistor T4 is larger than the drive voltage V.sub.O, the transistors T5 and T6 are turned-on if the collector voltage of transistor T4 becomes equal to the drive voltage V.sub.O, whereby the transistor T1 becomes a conductive state, and the excitation current I.sub.B flows from the inductive load L.
In such a prior art, since the constant-current source C1 and the diodes D1 and D2 are inserted between a power source V.sub.CC and the output terminal S1, an upper limit of the drive voltage V.sub.O that the idling loop is operated is "V.sub.CC -V.sub.SAT(C1) -2V.sub.F "(V.sub.SAT(C1) : a saturation voltage of the constant-current source C1). On the other hand, when the excitation current I.sub.B flows, the transistor T3 is not turned-on, and thus, an upper limit of a dynamic range of the drive voltage V.sub.O is more than "V.sub.CC ". Accordingly, there is a possibility that the transistor T1 is not suitably controlled, and thus, a negative feedback operation becomes unstable. That is, since the phase of the current flowing through the inductive load L is delayed by 90 degrees from the drive voltage V.sub.O, it is necessary to turn-on the transistor T1 such that the current I.sub.B can flow during a period shown by oblique lines in FIGS. 6A and 6B. Accordingly, if the drive voltage V.sub.O is too large, the transistor T1 is not suitably turned-on, and therefore, the negative feedback operation becomes unstable.